Theoretical study on the insertion reaction of the stannylenoid H2SnLiF with X-H bonds (X = N, O, F)

The insertion reactions of p-complex (RP) and three-membered ring configuration (RS) of stannylenoid H2SnLiF with NH3, H2O and HF have been studied theoretically by quantum chemical calculation. The structures of reactants, precursors, transition states, intermediates and products have been fully optimized at the M06-2X/def2-TZVP level. The single point energy of all fixed points were calculated using the QCISD method. The calculation results show that the three-membered ring configuration is easier to conduct the insertion reaction. Comparing the reaction energy barriers of RP, RS to NH3, H2O and HF, we found that the difficulty of the insertion reaction is NH3 > H2O > HF. The solvent corrected calculation results show that in THF, the reaction energy barrier of RP is lower than that in vacuum, while the reaction energy barrier of RS is higher. This work provides theoretical support for the reaction properties of stannylenoid.

Theoretical study on the insertion reaction of the stannylenoid H 2 SnLiF with X-H bonds (X = N, O, F)

Theoretical methods
The M06-2X [51] methods with the def2-TZVP basis set were used to optimize the geometries of fixed points on the potential energy surface, and then the harmonic vibration frequency was calculated at the same level of theory to represent the minimum or first order saddle point of the optimized geometry.In order to improve the processing of electronic correlation, the QCISD [52] method was used to calculate the single point energy of all fixed points at the def2-TZVP theoretical level.The mechanism of the insertion reaction was verified by the analysis of the intrinsic reaction coordinate (IRC) of the possible transition state (TS).In order to consider the solvent effect, the SMD model [53] was used to calculate the geometry and energy of the fixed point in the insertion reaction, and tetrahydrofuran (THF) solvent was selected (dielectric constant ε = 7.4257).The Multiwfn and VMD softwares were used to draw frontier molecular orbitals [54][55].
All calculations were performed using the Gaussian 09 [56] series of programs.

Results and discussion
The previous work [57] has shown that when stannylene H 2 Sn: and alkali halide LiF coexist, they are difficult to separate from each other and will exist in the form of stannylenoid H 2 SnLiF.The most stable configuration of H 2 SnLiF at HF/3-21G level was found to be the p-complex configuration, followed by the three-membered ring configuration, and the energy barrier for the conversion between the two configurations is relatively large and difficult to convert.The M06-2X/def2-TZVP calculations in present work predicated similar results with Ref. [57].There are three types of structures, namely p-complex configuration (RP), three-membered ring configuration (RS) and "classical" tetrahedral configuration for H 2 SnLiF, which energies at the QCISD/def2-TZVP//M06-2X/def2-TZVP level are -322.1117,-332.0977 and -322.0687a.u., respectively.Therefore, the p-complex configuration and three-membered ring configuration are two basic structures of H 2 SnLiF.On the other hand, the energy barrier for RP to RS is 111.96kJ/mol, and the energy barrier for RS to RP is 75.62 kJ/mol, which means it is difficult to convert into each other for RP and RS.Therefore, in this work, we use the p-complex configuration (RP) and three-membered ring configuration (RP) to explore the insertion reaction with NH 3 , H 2 O, and HF.The insertion reaction of stannylenoid H 2 SnLiF with X-H bonds (X = N, O, F) can be described as the following equation: Based on the calculated results, it can be reasonably predicted that the first step of the reaction between H 2 SnLiF and XH n is to form the precursor complex (Q).And there is a transition state (TS) and an intermediate (IM) along the reaction potential energy surface, and these structures connect reactants and products respectively.The Figure 1 shows the frontier molecular orbitals of p-complex configuration (RP) and three-membered ring configuration (RS) of H 2 SnLiF.The Figures 2 and 3 show the reaction process and the relative energy and structural parameters of each stationary point of the potential energy surface (relative to the corresponding reactants).Figures 4 and 5 show changes in energy and bond distance of reaction coordinate of RP and RS with XH n insertion reaction.Tables S1-S6 in Supporting information shows the charge changes of each atom in the insertion reaction of RP, RS with XH n , respectively.Table S7 and S8 in Supporting information shows the energy of each stationary point on the potential energy surface of RP and RS insertion reaction.

Insertion reaction of p-complex configuration (RP)
The p-complex structure (RP) of H 2 SnLiF can be regarded as a stannylene complex with ionic compound LiF (Figure 1).RP is formed by contributing some electrons of F atom in LiF to the unoccupied p orbital of Sn atom in singlet H 2 Sn.In fact, there is also weak interaction between Li atom (with positive charge) and two H atoms (with negative charge).The highest occupied molecular oibital (HOMO) of RP is mainly composed of σ orbital on Sn atoms, so the RP shows obvious nucleophilicity in the σ orbital direction.The insertion reaction between RP and X-H bonds is caused by the interaction between the σ orbitals occupied by Sn atom and the s orbitals on H atoms of X-H bonds.When the H atom further approaches the Sn atom through the electrostatic interaction, the X end of the X-H bond also interacts with the Sn atom.

The structures and energies of the precursor complexes
When XH n is close to RP, the H end of X-H bonds and the σ orbital of Sn atom interaction forms precursor PQ N , PQ O, and PQ F .It can be seen from Figure 2 that the H 2 SnLiF part of precursor PQ N , PQ O, and PQ F structure has little change compared with the reactant.The Sn-H distances are 3.297, 3.019, and 2.651 Å, and the distances of Sn-N, Sn-O and Sn-*F are 2.708, 2.763, and 3.579 Å, respectively.The relative energies (shown in Figure 2) of PQ N , PQ O and PQ F are -29.39,-17.01, and -15.75 kJ/mol, respectively.

The structures and energies of the transition states
As XH n further approaches RP, the interaction between XH n-1 and the p orbital of Sn atom reaches the transition state RTSXH n .As shown in Figure 2, the bond lengths of Sn-H a , Sn-H b, and Sn-H c are 2.810, 1.772, and 1.759 Å, and the  distances of Sn-N, Sn-O and Sn-F are 2.263, 2.223, and 2.241 Å respectively.Compared with PQ X , the bond lengths of Sn-H and Sn-X are significantly shorter.It can be seen from Tables S1-S3 in Supporting information that in this process, the natural charge of Sn atom increased from 1.438 to 2.242, 2.288, and 2.288, when X = N, O, F, respectively, and the natural charge of X and H atoms decreased significantly, which indicates that RP showed nucleophilic behavior in insertion reaction.Frequency analysis calculations performed at the M06-2X/def2-TZVP level of theory show that PTSX n has a unique imaginary frequency (1413.7,1438.8,1393.9 i cm −1 ).IRC analysis shows that PTSX n is the real transition state in the insertion reaction of H 2 SnLiF with XH n , and correctly connects the precursor complex PQ X and the intermediate PIMXH n .The relative energies of PTSXH n are 247.41,201.44, and 147.22 kJ/mol, when X = N, O, F, then the potential energy barrier of the insertion reaction are 276.80,218.45, and 162.97 kJ/mol, respectively.

The structures and energies of the insertion intermediates
As the reaction proceeds, after PTSXH n , the intermediate PIMXH n is formed with the breaking of X-H bond and the generation of Sn-X and Sn-H bonds.In PIMXH n , compared with PTSXH n , the X-H bond was extended to 2.900, 2.900, and 2.787 Å, respectively, indicating that it was gradually broken during the formation of intermediates.The Sn-X and Sn-H distances were shortened to 2.058, 2.004, 1.962 and 1.703, 1.706, and 1.697 Å, respectively, indicating that the Sn-X and Sn-H bonds were gradually formed.While the Sn-F and Sn-Li bonds were significantly prolonged, indicating that the LiF part was gradually separated.At this time, the natural charge of the Sn atom increases, and the natural charge of X atom and H atom decreased significantly, indicating that the nucleophilic reaction continued to occur.The relative energies of PIMXH n are 41.58, 1.21, and -54.09 kJ/mol, respectively.

The structures and energies of the products
As the reaction proceeds, when LiF is completely separated from Sn, the product PPXH n is obtained.It can be found from Figure 2 that the structures of PPNH 3 , PPH 2 O and PPHF are very similar, and they are all in four-coordinate configuration.Their relative energies are 96.52,68.74, and 28.96 kJ/mol, respectively, indicating that the insertion reaction of RP is endothermic.As aforementioned, the insertion reaction of RP and XH n is endothermic.The three products are similar.The order of the energy barrier is NH 3 > H 2 O > HF, which indicates that HF is more prone to the insertion reaction, followed by H 2 O and NH 3 .

Mechanism of the insertion reaction
In order to explain the mechanism of the insertion reaction of RP with XH n , we carried out the intrinsic reaction coordinate (IRC) analysis based on the optimized structure of transition states (PTSNH 3 , PTSH 2 O, PTSHF).Since the mechanism is similar, the reaction of RP with NH 3 is chosen as an example to describe.
Figure 4 shows the changes of energy and Sn-H a , Sn-N and N-H a bond distances along the reaction coordinates.It can be seen from the figure that in the region of the reaction coordinate (10-0), the energy rises sharply and reaches the maximum energy at 0 point, which is in the transition state (PTSNH 3 ).In this region, the distance between Sn-H a and Sn-N is continuously shortened, which means that NH 3 is constantly approaching RP and has the tendency to form new bonds.The distance between N-H a bonds began to lengthen, and H a tended to leave NH 3 .After 0 point, the energy of the reaction system begins to decrease.The distance between Sn-H a and Sn-N gradually decreases to a constant, which indicates that new Sn-H a and Sn-N bonds have been formed, accompanied by the fracture of N-H a bonds.

Insertion reaction of three-membered ring configuration (RS)
It can be seen from Figure 1 that the components of HOMO of RS are mainly concentrated on Sn atom and two H atoms, while the components of LUMO are mainly concentrated on Li atom, and a small part of them are concentrated on the p orbitals of Sn atom.Since the F atom gives electrons to the p orbital on the Sn atom, the p orbital on the Sn atom is not empty.When the insertion reaction occurs, the H end of the X-H bond first attacks the Sn atom σ orbit, and then close to RS, and the electron is partially transferred to the s orbital of H atom.Then, the X end of the X-H bond interacts with the p orbital on the Sn atom in RS to complete the reaction.

The structures and energies of the precursor complexes
At the beginning of the reaction, X-H is close to RS, and the H end of X-H bond is connected with σ Orbital of Sn atom.The orbits combine to form the precursor SQ x .From the Figure 3, the distances between Sn-X and Sn-H are 2.601, 2.711, 2.996 Å and 3.149, 3.032, 2.677 Å respectively, and the relative energies of SQ N , SQ O and SQ F are-38.47, -23.56, and -16.25 kJ/mol respectively, which indicates that this is an exothermic process.

The structures and energies of the transition states
As XH n approaches RS further, the X terminal interacts with the p orbital on the Sn atom on the back of the F atom in RS.The interaction between XH n and Sn atom weakens the X-H bond.The reaction reached the transition state STSXH n .As shown in the Figure 3, the Sn-H distance was 1.853, 1.843, 1.826 Å and the Sn-X distance was 2.259, 2.228, and 2.268 Å respectively.Compared with SQ X , the Sn-H and Sn-X distances were significantly shortened.It can be seen from the Tables (S4-S6) that the natural charge of sn atom increased from 1.522 to 2.305, 2.334, 2.317 respectively, and the natural charge of X and H atoms decreased significantly, indicating that this is a nucleophilic process.Frequency analysis calculations performed at the M06-2X/def2-TZVP level of theory show that STSX n has a unique imaginary frequency (1582.0,1527.1, 1426.3 i cm −1 ).IRC analysis shows that STSX n is the real transition state in the insertion reaction of H 2 SnLiF with XH n , and correctly connects the precursor complex SQ X and the intermediate SIMXH n .The relative energy of STSXH n is 208.66,165.09, 114.86 kJ/mol respectively, so the potential barrier of RS insertion reaction is 247.13,188.65, 131.11 kJ/mol respectively.

The structures and energies of the insertion intermediates
After the transition state STSXH n , as the reaction proceeds, the X-H bond gradually breaks, the Sn-H and Sn-X bonds gradually form, forming the intermediate SIMXH n .At this time, the natural charge of the Sn atom increases, the natural charge of the X and H atoms decreases, the electrons of XH n attack the Sn atom as a nucleophilic reagent, the ternary ring in RS is destroyed, the distance between the Li atom and the F atom is shortened, and there is a tendency to leave.The relative energy of SIMXH n is 6.69, -34.67 and -90.90 kJ/mol respectively.

The structures and energies of the insertion products
As the reaction proceeds, LiF leaves and the product SPXH n is obtained.It can be seen from the Figure 3 that the structures of SPNH 3 , SPH 2 O and SPHF are very similar, and are the same as the insertion reaction products of RP.Their relative energies are 59.71, 31.94, and -7.86 kJ/mol respectively, indicating that the insertion reaction of RP with NH 3 and H 2 O is endothermic, and the reaction with HF is exothermic.
It can be seen from the above that the insertion reaction products of RS and XH n are the same as those of RP reaction.The order of the insertion reaction barrier size is NH 3 > H 2 O > HF, which is the same as that of RP reaction.And the reaction barrier of RS and RP are compared respectively.We find that the energy barrier overcome by RS is lower and the reaction is easier to occur.

Mechanism of the Insertion reaction
In order to explain the mechanism of the insertion reaction of RS with NH 3 , H 2 O and HF, we carried out the intrinsic reaction coordinate (IRC) analysis based on the optimized structure of transition states (STSNH 3 , STSH 2 O, and STSHF).Taking the reaction with NH 3 as an example, the reaction mechanism of RS and XH n (X = N, O, F, n = 3, 2, 1) will be summarized below.
Figure 5 shows the change of reaction coordinates along the reaction path of Sn-H d , Sn-N and N-H d distances with energy.In the reaction between RS and NH 3 , it can be seen from the figure that in the region of the reaction coordinate (20-0), the energy rises sharply and reaches the maximum energy at point 0. At this time, it is in the transition state (STSNH 3 ).In this region, we can see that the distance between Sn-H d and Sn-N is continuously shortened, which means that NH 3 is constantly approaching RS, the distance between N-H d bonds begins to lengthen, and H d has a tendency to leave NH 3 .After 0, the energy of the system begins to decrease, and the system gradually becomes stable.The distance between Sn-H d and Sn-N is shortened to a certain extent and becomes stable, which indicates that new Sn-H d and Sn-N bonds have been formed.The distance between N-H d bonds is extended to a certain extent.At this time, H d atom has been combined with Sn atom and LiF has been separated.

Solventing effect
In order to study the effect of solvent on the insertion reaction, we use the SMD model to set the insertion reaction to occur in THF solvent.The calculated results show that for the reaction involving RP, the inserted reaction barrier in THF solvent is 263.11, 190.19, 121.97 kJ/mol, which is lower than the reaction barrier in vacuum.For the reaction involving RS, the inserted reaction barrier in THF solvent is 273.07,203.28, 141.56 kJ/mol, which is increased compared with the reaction barrier under vacuum.This shows that for RS, the reaction in THF solvent is unfavorable.For RP and RS, the order of difficulty of insertion reaction in THF solvent is N > O > F, and the reaction barrier of RS is lower than that of RP, so it is easier to conduct insertion reaction.

Conclusion
In this paper, the insertion reactions of the p-complex configuration (RP) and the ternary ring configuration (RS) of stannylenoid H 2 SnLiF with NH 3 , H 2 O, and HF have been studied theoretically.According to the calculation results, we know that the reaction products of RP and RS are the same, and through the calculation of the reaction energy barrier, the insertion reaction potential barrier of RS is lower than that of RP, which means that RS is easier to react.Comparing the reaction barrier of RP and RS with NH 3 , H 2 O, and HF, we found that the difficulty of insertion reaction is NH 3 > H 2 O > HF, regardless of the configuration of p-complex or three-membered ring.According to the calculation results, in THF solvent, the insertion reaction of RP with NH 3 , H 2 O, and HF is favorable, while the insertion reaction of RS with NH 3 , H 2 O, and HF is unfavorable.In addition, for RP and RS, the difficulty order of insertion reaction in THF solvent is N > O > F, and the reaction barrier of RS is lower than that of RP, so it is easier to conduct insertion reaction.We hope that the calculation results given in this paper are satisfactory and can make some useful predictions for the experiment.

Figure 2 .
Figure 2. Schematic diagram of relative energy and structural parameters of each stationary point of the insertion reaction channel between RP and NH 3 , H 2 O and HF and the reaction potential energy surface under the M06-2X method (the values in parentheses are the results obtained in THF solvent, bond length is in Å, bond angle is in degree).

Figure 3 .
Figure 3. Schematic diagram of relative energy and structural parameters of each stationary point of the insertion reaction channel between RS and NH 3 , H 2 O and HF and the reaction potential energy surface under the M06-2X method (the values in parentheses are the results obtained in THF solvent, bond length is in Å, bond angle is in degree).

Figure 4 .
Figure 4. Changes in energy and bond distance of reaction coordinate of RP and XH n (X = N, O, F, n = 3, 2 1) insertion reaction.

Figure 5 .
Figure 5. Changes in energy and bond distance of reaction coordinate of RS and XH n (X = N, O, F, n = 3, 2, 1) insertion reaction.

Table S1 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RP and NH 3 insertion reaction at M06-2X/def2-TZVP level.

Table S2 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RS and H 2 O insertion reaction at M06-2X/def2-TZVP level.

Table S3 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RP and HF insertion reaction at M06-2X/def2-TZVP level.

Table S4 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RS and NH 3 insertion reaction at M06-2X/def2-TZVP level.

Table S5 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RS and H 2 O insertion reaction at M06-2X/def2-TZVP level.

Table S6 .
Natural bond orbital analysis (NBO) of the stagnation points on the potential energy surface of RS and HF insertion reaction at M06-2X/def2-TZVP level.

Table S7 .
Energy of each stationary point on the potential energy surface of RP insertion reaction (kJ/mol).

Table S8 .
Energy of each stationary point on the potential energy surface of RS insertion reaction (kJ/mol).